1. Field of the Invention
This invention relates to a wave reception apparatus of a motor vehicle and more particularly, to an apparatus having high sensitivity with using a low gain antenna element.
2. Description of the Prior Art
It is known that defogging heater wires and antenna conductors for receiving FM radio waves are formed on a rear window glass of a motor vehicle and are connected to each other through a stub in order to receive both AM and FM radio waves. This known system is disclosed in U.S. Pat. No. 4,063,247.
The magnitude of an electromotive force e.sub.0 for AM radio waves induced within such an antenna depends upon the area of the window glass. Induced e.m.f. e.sub.0 becomes smaller as the area of the window glass reduces.
On the contrary, a capacitance of a stray capacity of the antenna to a ground, valued when the antenna is viewed from the input terminal of a tuner, does not reduce in proportion to the area of the window glass, so that if the window glass becomes small, the AM radio waves are not received well in an area where the radio field strength is rather weak. Thus, in a recent broadcast reception system for the vehicle, the induced e.m.f. e.sub.0 is supplied to a receiver through a preamplifier.
A well-known antenna pattern shown in FIG. 6 comprises a plurality of defogging heater wires 2 arranged horizontally and divided into two groups, an upper group and, a lower group. The left ends of upper and lower groups of the heater wires 2 are connected to buses 3 and 4, respectively. The right ends of heater wires 2 are connected to a bus 5, so that the upper group and the lower group are power-supplied in series.
A feeding point 6 provided in the bus 5 is connected to a pre-amplifier 9. Input signals induced in the heater wires 2, which are used as an antenna for receiving the AM radio waves, are amplified by the pre-amplifier 9, and then fed to a tuner through a feeder cable 7, such as a coaxial cable.
A heating current to the heater wires 2 flows through choke coils 11a and 11b which are magnetically coupled with each other, power lines 12 and 13, and the buses 3 and 4. The choke coil 11a connected to a power source +B, and the choke coil 11b connected to the ground are magnetically coupled negatively to each other, so that a core for the coils 11a and 11b is not easily saturated. A reception signal induced on the heater wires 2 are not conducted therefrom to the power source or the ground due to high impedance of the choke coils 11a and 11b in a radio wave band, which improves reception efficiency. A decoupling capacitor 14 is connected to the power source circuit, so that the reception signal is not interfered with noises generated by the power source.
FIG. 7 shows an equivalent circuit of the antenna circuit of FIG. 6, and notations used in FIG. 7 are as follows.
e.sub.0 : induced electromotive forces of the heater wires 2 PA0 C.sub.1 : an active capacitance of the antenna PA0 C.sub.2 : a reactive capacitance of the antenna PA0 C.sub.L : a stray capacitance of the power lines 12 and 13 PA0 L.sub.x : an equivalent inductance of the choke coils 11a and 11b (equal to one-half of the inductance of each choke coil) PA0 C.sub.d : a self-capacitance of the choke coil PA0 C.sub.k : a stray capacitance of the feeder cable 7
As the vehicle runs, receiving conditions of a radio receiving system mounted on the vehicle vary, that is, the antenna formed on the glass of the rear window is situated in various electric field intensities. Therefore if the amplifier 9 is used to set a high reception gain, the reception signals are apt to be interfered with thermal noises of the amplifier 9, and noises included in the reception signals will be amplified. On the contrary, if radio waves are received near an antenna of a broadcast station, broadcasting voices by radio are distorted due to excessive amplification by the amplifier.